Through the years, a variety of techniques have been developed for delivering nutrients to growing plants and for extending or delaying the release of nutrients from a fertilizer.
Fertilizer is often applied as a formulated (N—P—K) solid, granule or powder, or sometimes as a liquid, to an area to be fertilized. There are basically two types of fertilizers, water-soluble fertilizers and “slow-release” fertilizers. While water-soluble fertilizers are generally less expensive than slow-release fertilizers, they have the disadvantage of leaching nutrients very quickly into and through the soil. Some solid, water-soluble fertilizers can be converted into slow-release fertilizers by employing various coatings. Alternatively, a reduction in nitrogen availability also can be obtained by using enzyme inhibitors. Slow-release fertilizers are designed to release nutrients to plants or soil over an extended period of time, which is more efficient than multiple applications of water-soluble fertilizers. Therefore, slow-release fertilizers (also referred to as controlled release or extended release) minimize the frequency with which plants must be fertilized, as well as reduce or minimize leaching.
Urea-formaldehyde (UF) condensation products are widely used as slow-release nitrogen fertilizers for crops, ornamental plants and grasses. Urea-formaldehyde fertilizer materials also can be supplied either as liquids or as solids and are the reaction products of urea and formaldehyde. Such materials generally contain at least 28% nitrogen, largely in a water-insoluble, slowly available form.
Extended release UF fertilizers (ureaform) can be prepared by reacting urea and formaldehyde at an elevated temperature in an alkaline solution to produce methylol ureas. The methylol ureas then are acidified to polymerize the methylol ureas to methylene ureas, which increase in chain length as the reaction is allowed to continue. These methylene urea polymers normally have limited water solubility, and, thus, release nitrogen throughout an extended period. Such UF fertilizers usually include a mixture of methylene urea polymers generally have a range of molecular weights and are understood to be degraded slowly by microbial action into water-soluble nitrogen. UF fertilizers are usually categorized by the amount and the release characteristics of their water insoluble nitrogen.
U.S. Pat. No. 4,089,899 (the disclosure of which is incorporated herein by reference) describes a solid, controlled release nitrogen fertilizer of the ureaform type, which consists essentially of only two nitrogen fractions: water-soluble nitrogen and cold water insoluble nitrogen.
U.S. Pat. No. 3,677,736 (the disclosure of which is incorporated herein by reference) describes a urea-formaldehyde fertilizer suspension. Other disclosures of urea-formaldehyde fertilizer compositions, both liquid and solid forms, include U.S. Pat. Nos. 4,378,238; 4,554,005; 5,039,328; 5,266,097; 5,674,971; 6,432,156; 6,464,746; 6,900,162; 6,936,573 and 6,936,681, the disclosure of which are all incorporated herein by reference.
Granular nitrogen-containing fertilizers have been produced commercially by a variety of techniques using water-soluble nitrogen products, such as urea, potassium nitrate, and ammonium phosphate. The practical advantages of handling, blending, and storing such fertilizer granules are known and well documented. The preparation of granular fertilizers using slow-release UF fertilizers also has been described in the prior art.
Recently, it has been proposed to use poly(aspartic acid) to enhance plant nutrient uptake. See U.S. Pat. No. 5,593,947 (incorporated herein by reference). For example, U.S. Pat. No. 5,350,735 (incorporated herein by reference) discloses ammoniacal nitrogen, nitrate nitrogen and urea nitrogen combined with poly(aspartic acid) to form a fertilizer. However, the nitrogen disclosed in U.S. Pat. No. 5,350,735 is a water-soluble form of nitrogen. Other similar patents are U.S. Pat. Nos. 5,783,523; 5,814,582; 5,854,177; 5,861,356 and 5,935,909 (all of which are incorporated herein by reference). However, tests with water-soluble forms of nitrogen in combination with poly(aspartic acid), or hydrolyzed forms of poly(aspartic acid), have not proven to sufficiently enhance plant growth so as to make the use of this composition economically feasible.
Accordingly, there is a need for a fertilizer that releases nitrogen slowly and improves plant, crop and turf growth relative to conventional slow-release nitrogen fertilizers or relative to thermal polyaspartate alone.